1. Field of Invention
This invention pertains to the operation of a helical scan recorder, and particularly for determining parameters of the helical scan recorder for enhancing performance thereof.
2. Related Art and Other Considerations
Numerous prior art patents and publications teach recording and reading of information stored in helical stripes (or "tracks") on magnetic storage media (e.g., magnetic tape). In a helical scan arrangement, travelling magnetic tape is at least partially wrapped around a rotating drum (or scanner) so that heads (both write head(s) and read head(s)) positioned on the drum are contiguous to the drum as the drum is rotated.
One or more write heads on the drum physically record data on the tape in a series of discrete tracks oriented at an angle with respect to the direction of tape travel. As used herein, track or stripe "pitch" means a distance between centerlines of two adjacent tracks, the centerlines of the tracks extending along the direction of head travel and the distances therebetween being taken perpendicularly to the centerlines. In a dual azimuth system, track pitch equates to the width of a track. The data on the track is formatted, prior to recording on the tape, to provide sufficient referencing information to enable later recovery during readout by one or more read heads.
Examples of helical scan recorders are shown, inter alia, in the following U.S. patents (all of which are incorporated herein by reference):
U.S. Pat. No. 4,835,628 to Hinz et al. PA1 U.S. Pat. No. 4,843,495 to Georgis et al. PA1 U.S. Pat. No. 5,065,261 to Hughes et al. PA1 U.S. Pat. No. 5,068,757 to Hughes et al. PA1 U.S. Pat. No. 5,142,422 to Zook et al.
On a drum of a helical scan recorder, the write head(s) are distanced from the read head(s) both by a radial distance and an axial distance (the axial distance being taken along the major axis of the drum). The separation of the write head and read head along the major axis of the drum is herein denoted as the "axial offset" or "axial offset distance". Although a helical scan recorder is intended to be manufactured to have a specification axial offset distance, it generally turns out that the drum of a helical scan recorder as manufactured has an actual axial offset distance which varies from the specification axial offset distance. As used herein, "axial offset variance" means the differential between (1) a desired (e.g., specification or reference) axial offset distance by which a write head is supposed to be separated from a read head on the drum along the drum axis, and (2) an actual axial offset distance by which a write head is actually separated from a read head on the drum along the drum axis.
To the extent that axial offset variance has been measured in the prior art, such measurements have taken the form of imprecise gauging with the use of optical measurement devices such as a high power microscope. However, the margin of error of such measurement devices is greater than the precision required for use in a helical scan recorder.
Axial offset variance has importance for a number of reasons. For example, axial offset variance is a factor which complicates write splice operations. In a write splice, the recorder must start recording exactly at a point ("splice location") at which the previous recording had stopped. To maximize media usage, the junction of new data to old data must be seamless, so that track pitches are continuous.
If there is no axial offset variance (e.g., axial offset variance=0), track pitch uniformity can easily be obtained at the write splice location. However, even a small axial offset variance (for example, two microns) will result in nonuniformity of track pitch at the write splice location. Subsequent read operations in the neighborhood of the write splice location can cause servoing problems, particularly if several splices are close together.
Axial offset variance also can be a factor in determining linear tape speed in certain helical scan recorders, such as a capstanless helical scan recorder. In this regard, see simultaneously-filed U.S. patent application Ser. No. 08/150,726 (attorney docket 1300-135) of Georgis and Zweighaft entitled "Method And Apparatus For Controlling Linear Tape Speed In A Helical Scan Recorder" (incorporated herein by reference) now abandoned.